1. Technical Field of Application
The present invention relates to multiple-frequency modulation type mode-locked laser devices which generate high-speed optical pulses, for use as high-speed optical pulse sources in the fields of optical communications, optical measurement, or the like.
2. Related Art
Mode-locked laser devices are able to generate ultrashort optical pulses (on the order of femtoseconds) and transform-limited (TL) optical pulses (optical pulses which minimize the time-bandwidth product) which are suited to long-distance optical transmissions. In order to take advantage of these features, research has progressed with an eye towards applications in the fields of high-capacity long-distance optical communications or high-speed optical measurement (see S. Kawanishi et al., "100 Gbit/s, 200 KM OPTICAL TRANSMISSION EXPERIMENT USING EXTREMELY LOW JITTER PLC TIMING EXTRACTION AND ALL-OPTICAL DEMULTIPLEXING BASED ON POLARIZATION INSENSITIVE FOUR-WAVE MIXING", Electron. Lett., Vol. 30, No. 10, 1994, pp. 800-801).
FIG. 11 shows an example of the composition of a mode-locked laser device of the prior art (see R. P. Davey et at., "HIGH-SPEED MODE-LOCKED, TUNABLE, INTEGRATED ERBIUM FIBRE LASER", Electron, Lett., 1992, 28, pp. 482-484).
According to this diagram, the mode-locked laser device is composed of a drive signal generation means 10a and a ring cavity 20. The drive signal generation means 10a amplifies an oscillator output [1] from an oscillator 111 which oscillates at a set frequency f.sub.m with a voltage amplification means 12, superimposes a DC voltage V.sub.b provided by a DC voltage source 14 with a DC voltage superimposing means 13, and outputs the resulting drive signal [2]. The ring cavity 20 is composed of an optical modulation means 21, an optical amplification means 22 which amplifies the modulated optical pulse, an optical isolator 23 which fixes the direction of propagation of the optical pulse and isolates the light which has been reflected back, an optical filter 24 which determines the oscillation wavelength within the width of the gain spectrum of the optical amplification means 22, a light splitting means 25 which guides the amplified optical pulse (mode-locked laser output beam [5]) to the outside, an optical delay means 26 which adjusts the cavity length, and the optical coupling means 27, 27, 27, . . . which optically couples each of the means in a ting shape. The optical modulation means 21 modulates either the loss or phase of the light based on the frequency f.sub.m of the drive signal [2] received from the drive signal generation means 10a.
As optical modulation means 21, in principle, a modulator utilizing the electro-optical effects of LiNbO.sub.3 or the like, a semiconductor laser amplifier, an electroabsorption optical modulator, or others may be used.
As optical amplification means 22, in principle, rare earth-doped fiber amplifiers doped with rare earth elements such as Er or Nd, or semiconductor laser amplifiers may be used. For the rare earth-doped fiber amplifiers, regarding the manner of injection of the pumped light, them are forward pumping, backward pumping, and bi-directional pumping.
As optical coupling means 27, optical fiber, channel-form optical waveguides formed on flat substrates (see Y. Hibino et al., "SILICA-BASED OPTICAL WAVEGUIDE RING LASER INTEGRATED WITH SEMICONDUCTOR LASER AMPLIFIER ON Si SUBSTRATE", Electron. Lett., 1992, 28, pp. 1932-1933), or others may be used.
Below, the theory behind mode-locked laser devices of the prior art will be explained with reference to FIGS. 11 and 12. In these diagrams, [1] is an oscillator output, [2] is a drive signal, [3] is a optical modulator characteristic, [4] is an optical modulation waveform, [5] is a mode-locked laser output waveform, and [6] is a spectrum.
When a Mach-Zehnder optical intensity modulator is used as the optical modulation means 21, in general, the transmission intensity characteristics of the optical modulator can be described as follows: ##EQU1## Here, V.sub.0 is the half-wave voltage, V is the drive signal voltage, and V.sub.b is a constant.
The oscillator output [1] of the frequency f.sub.m is amplified by the voltage amplification means 12 so that the peak-to-peak voltage V.sub.p--p is below V.sub.0 (within the voltage range of the maximum-minimum values of the optical modulator characteristic [3]), and the DC bias voltage V.sub.b is superimposed on it by the DC voltage superimposing means 13, resulting in the drive signal [2]. This drive signal [2] can be described as follows: ##EQU2##
If the oscillator output [1] has a sufficiently large amplitude, then the voltage amplification means 12 is unnecessary. Additionally, if the DC bias voltage V.sub.b of the optical modulator [3] is sufficiently close to zero, then the DC voltage superimposing means 13 and the DC voltage source 14 are unnecessary. By activating the optical modulation means 21 with this drive signal [2], the optical modulation waveform [4] becomes as follows: ##EQU3## Hereinafter, the frequency f.sub.m of the drive signal [2] which drives the optical modulation means 21 will be referred to as the drive frequency.
The optical path length R of the ting cavity, taking L as the physical length and n as the index of refraction of each element in the ring cavity, can be described as the sum of the products of all of the respective physical lengths L.sub.i and indices of refraction n.sub.i (the respective optical path lengths), as follows: ##EQU4## In this ring cavity, multiple longitudinal modes exist with a frequency spacing of f.sub.r (=c/R wherein c is the speed of light). When the frequency spacing f.sub.r is equal to f.sub.m, that is: ##EQU5## then as shown in FIG. 12 [6], the phases of all of the longitudinal modes at a frequency spacing of f.sub.r match, resulting in a mode-locked oscillation state, and as shown in [5], an optical pulse train of repetition frequency F.sub.rep =f.sub.m is obtainable.
On the other hand, if f.sub.m .noteq.f.sub.r, then an optical delay means 26 is provided within the ring cavity as shown in FIG. 11, and thus the optical path length R of the ring cavity is adjusted. The center of this spectral envelope becomes the central wavelength (frequency v.sub.0).
Additionally, the optical pulse which minimizes the product of the pulse width .DELTA.t and the oscillation spectrum width .delta.v determined by the envelope of the multiple longitudinal mode spectrum (time-bandwidth product) is called the transform limited (TL) pulse.
Furthermore, if the drive frequency f.sub.m is an integral multiple of the frequency spacing f.sub.r, then as a condition of harmonic mode-locking, the following must be true: ##EQU6## and an optical pulse train of repetition frequency F.sub.rep =f.sub.m =N.multidot.f.sub.r is obtainable. In this case, N is a natural number. Therefore, with mode-locked laser devices of the prior art, the repetition frequency f.sub.rep and the drive frequency f.sub.m of the output beam are equal.
The pulse width .DELTA.t of this mode-locked laser output beam [5], according to mode-locking theory, may be described as follows (see A. E. Siegman, "LASERS", University Science Books, p. 1064): ##EQU7## In this case, .gamma. is the gain coefficient of the optical amplification means 22, .DELTA.m is the peak-to-peak amplitude modulation of the optical modulation means 21, and .DELTA.f.sub.a is the bandwidth of the optical amplification means 22. From this, it is known that the pulse width .DELTA.t of the mode-locked laser output beam [5] is proportional to 1/(f.sub.m .DELTA.f.sub.a).sup.1/2. Therefore, in order to decrease (or increase) the pulse width .DELTA.t, the drive frequency f.sub.m of the optical modulation means 21 should be made higher (or lower). That is, the gate width of the optical modulation means 21 should be made narrower (or wider).
The gate width of the optical modulation means 21 mentioned here corresponds to the repeated full-width at half-maximum of the optical modulation waveform [4].
Therefore, with mode-locked laser devices of the prior art, the pulse width .DELTA.t depends on the repetition frequency F.sub.rep (=drive frequency f.sub.m). The relationship between the pulse width .DELTA.t and the repetition frequency F.sub.rep which is determined by Equation (7) is shown in FIG. 13.
Furthermore, since the pulse width .DELTA.t also depends on the bandwidth .DELTA.f.sub.a determined by the optical filter 24, the relationship between each bandwidth is also shown.
Generally, for optical soliton transmission, an optical pulse with a specified pulse width is required for a given repetition frequency, for example, taking .DELTA.t=10 ps for F.sub.rep =10 GHz.
Furthermore, for optical measurement techniques such as optical sampling, an ultrashort pulse (on the order of a few ps or less) is required for a low repetition frequency (on the order of tens of KHz) (see Yamabayashi et at., "OPTICAL SAMPLING USING A LiNbO.sub.3 WAVEGUIDE AND ULTRASHORT OPTICAL PULSES", Abstracts of Spring 1988 National Convention of the. Electronic Data Communications Society, B-671).
In the past, when the repetition frequency F.sub.rep was fixed in this way, the pulse width .DELTA.t was controlled by changing .DELTA.f.sub.a in Equation (7), principally by adjusting the bandwidth of the optical filter 24 placed within the cavity. As shown by the points A, B, C, and D in FIG. 13, for a set repetition frequency F.sub.rep (in this case, 1.58 GHz), by increasing (or decreasing) the bandwidth .DELTA.f.sub.a, it is possible to decrease (or increase) the pulse width .DELTA.t.
The bandwidth .DELTA.f.sub.a of the optical amplification means 22 is restricted to the atomic linewidth .DELTA.f.sub.atom of the optical amplification medium. Therefore, with the pulse width control method of the prior art which used an optical filter 24, it was not possible to generate an optical pulse shorter than the pulse width (the shaded region in FIG. 13) without a filter (.DELTA.f.sub.a =650 GHz).
In contrast, there is a method wherein electric signals, having narrow pulse widths and being generated by a pulse generator or the like, are supplied to an optical modulation means 21 where their drive frequencies f.sub.m are made equally high, and thereby the pulse width .DELTA.t is made smaller for a set bandwidth .DELTA.f.sub.a. However, when generating electric signals with such narrow pulse widths, the jitter at the signal source such as a signal generator becomes large, and as a result, there is the drawback that the jitter in the mode-locked laser output beam becomes large as well.
Furthermore, for a given repetition frequency F.sub.rep, in order to obtain a TL optical pulse having a pulse width even smaller than the theoretical limit in this case, there is a method wherein adiabatic soliton compression is used on the optical pulse (see K. Suzuki et al., Tech. Digest OAA '93, TuD2, p. 314). However, with this method, since an adiabatic soliton compressor is required outside of the laser cavity, its composition is complicated.
Additionally, besides the reference material given above, there are the following papers:
(1) H. Takara et at., "GENERATION OF HIGHLY STABLE 20 GHz TRANSFORM-LIMITED OPTICAL PULES FROM ACTIVELY MODE-LOCKED Er.sup.3+ -DOPED FIBRE LASERS WITH AN ALL-POLARISATION MAINTAINING RING CAVITY", Electronics Letters, 22nd Oct. 1992, Vol. 28, No. 22, pp. 2095-2096. PA0 (2) H. Takara et al., "20 GHz TRANSFORM-LIMITED OPTICAL PULSE GENERATION AND BIT-ERROR-FREE OPERATION USING A TUNABLE, ACTIVELY MODE-LOCKED Er-DOPED FIBRE RING LASER", Electronics Letter, 24th Jun. 1993, Vol. 29, No. 13, pp. 1149-1150. PA0 (3) Japanese Patent First Publication No. 2-310982, laid open Dec. 26, 1990, "MODE-LOCKED FIBER LASER DEVICE", A. Takada, K. Hagimoto. PA0 (4) Japanese Patent First Publication No. 3-229478, laid open Oct. 11, 1991, "MODE-LOCKED LASER DEVICE", A. Takada, Y. Yamabayashi. PA0 (5) Japanese Patent First Publication No. 5-75194, laid open Mar. 26, 1993, "WAVELENGTH MULTIPLEXED MODE-LOCKED LASER DEVICE", M. Saruwatari, S. Kawanishi, H. Takara. PA0 (6) Japanese Patent First Publication No. 6-85366, laid open Mar. 25, 1994, "HARMONIC MODE-LOCKED LASER DEVICE", G. T. Harvey, L. F. Mollneur.